Packet-switched digital communication networks allow digital systems to communicate with each other. In these communication networks, data to be transmitted from one digital system to another is loaded into frames. The frames are then transmitted over the communication network to the destination digital system as packets. Often it is advantageous to compress the data before the data is transmitted as this reduces the amount of data to be transmitted over the communication network. It is also often desired to scramble or encrypt the data prior to transmission. When compressing, scrambling or encrypting data, it is necessary to ensure the transmitting digital system and receiving digital system remain in synchronization. Otherwise, the data encoding and decoding method will operate erroneously.
U.S. Pat. No. 5,130,993 to Gutman et al discloses a method for transmitting encoded data over unreliable networks wherein synchronization between the encoder and decoder is maintained. When encoded data is received by one system, the data is checked for transmission errors after the data has been decoded. If an error is detected, a reset signal is sent by the receiving system to the sending system to reset its encoder to a predetermined state known to the receiving system. The corrupted data can then be resent and decoded by the receiving system. In this manner, the synchronization of encoder and decoder can be maintained.
When the encoder is reset to a predetermined state, there is a reduction in encoding efficiency since the encoding algorithm must relearn its vocabulary. In prior art systems, such as that disclosed by Gutman, the predetermined state does not take advantage of encoding history between the encoder and decoder resulting in reduced system performance.
It is therefore an object of the present invention to provide a novel synchronization method and mechanism for maintaining synchronism between digital systems communicating over a lossy transmission media.